High strength niobium base alloys



This invention relates to niobium-base alloys, and particularly to niobium-base alloys containing titanium, zirconium, hafnium, and oxygen.

The trend toward higher operating temperatures in jet engines, missiles, gas turbines, and other heat engines has accelerated interest in the development of high temperature structural materials. At present, the primary 0.02% to 0.2% oxygen, preferably present as zirconia and hafnia, and the balance being essentially niobium with small amounts of incidental impurities. A particularly useful range of alloys of this invention, falling within the broad range just defined, comprises, by weight, from 0.6% to 2% titanium, from 0.6% to 2.5% zirconium, from .6% to 6% hafnium, from 0.04% to 0.2% oxygen, preferably present as zirconia and hafnia, and the balance essentially niobium with small amounts of incidental impurities.

Several of the alloys of this invention. were prepared by standard non-consumable arc melting techniques in which a protective atmosphere of argon was employed. The nominal compositions (with oxygen analysis) of the alloys prepared, together with certain of their properties, are presented in Table I which follows:

TABLE I Properties of Niobium-Base Alloys obstacle to improved performance in many power generation and propulsion systems is the unavailability of materials with sufficient high temperature strength. There is also a need for improved high temperature alloys in the construction of chemical reactors, oil refining equipment, and the like. The combination of workability at reasonable temperatures and strength at elevated temperatures has proved most difficult to achieve and much eifort has been devoted to attaining this end.

At the present time, molybdenum-base alloys are the only materials commercially available with strength properties comparable to the niobium-base alloys described in this application. However, these molybdenum-base alloys have much poorer oxidation resistance than pure niobium, they have approximately higher density, and they are quite difficult to fabricate. Maximum strength is achieved in the molybdenum-base alloys by carefully controlling working procedures in order to obtain a strain hardened material. However, if the alloy is heated to a temperature at which recrystallization occurs, as in welding, the strength will be drastically reduced by a factor of as much as 50%. The ductilebrittle transition temperature of molybdenum and commercially available molybdenum-base alloys is close to room temperature, and therefore, cold working is not practicable.

It is therefore the object of this invention to provide niobium-base alloys having good ductility at room temperature and high elevated temperature strength, the alloys including in predetermined amounts, the elements titanium, zirconium, oxygen and hafnium.

Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.

Accordingly, this invention is directed to niobium-base alloys suitable for use in the temperature range 1800 F. to 2400 F., and having reasonable ductility at room temperature. More particularly, the alloys of this invention comprise, by weight, from 0.5% to 10% titanium, 0.5% to 10% zirconium, 0.5% to 10% hafnium, from TABLE 11 Properties of Pure Niobium Test 0.2% Ultimate Per- Reduc- Composition, Temp., Yield Str. cent tion in Wt. percent F. Str. (p.s.i.) Elonarea (p.s.i.) gation (percent) Pure Nb RT 36, 000 48, 410 48. 0 63. 5 Pure Nb 2, 000 8, 10, 100 34. 0 100 The remarkable improvement in properties achieved at elevated temperatures by the specified] alloying additions to pure niobium, in accordance with this invention, is readily apparent.

It should be noted that the alloys of this invention include from .02% to 0.2% oxygen, and that this is an essential constituent of these alloys. While the function of the oxygen in these alloys is not fully understood, it is believed that predetermined amounts of ZrO and HfO and other oxides in finely dispersed condition are formed, and these oxides tend to confer additional strength upon the alloy.

In investigating the effect of oxygen in these alloys an alloy similar to alloy A of Table I was prepared except that the oxygen content was held to less than 0.04%, but more than 0.02%, by weight; that is, at the low end of the broad oxygen range. The nominal composition of this alloy and certain physical properties are presented in Table III.

Comparison of these data with the similar data for alloy A shows that the low-oxygen alloy is characterized by somewhat degraded properties. of oxygen, that is below 0.02%, the properties of the alloy will be lowered even more.

The titanium in these alloys is considered to provide improved oxidation resistance and workability within the ranges set forth. Another preferred range for titanium providing similarly improved properties is from 2.5% to 3.5%. The zirconium and hafnium in the alloys tends -to increase or improve their high temperature strength. For some purposes zirconium may be added in the range from 1.5 to 2.5%. Hafnium may be also added in amounts from 115% to 2.5% and from 2% to ..8%. The niobium employed in the .alloys v.of this invention will have a normal level of impurity content; that is, a total amount of about 1% or less.

The alloys of this invention are superior to the molybdenum-base alloys now commercially available in that they possess greater oxidation resistance at service temperature, they are relatively ductile and workable at room temperature, they have a favorable strength to weight ratio, and their strength is not dependent upon strain hardening.

It will be understoodby those skilled in the art :that although the present invention has been described in ,connection with preferred alloys, modifications .and varia- With lower amounts tions may be employed without departing from the underlying spirit and scope of the invention.

We claim as our invention: 1. A high temperature alloy consisting of from 0.5% 5 to titanium, from 0.5% to 10% zirconium, from 0.5% to 10% hafnium, from 0.02% to 0.2% oxygen, and the balance essentially niobium with small amounts of incidental impurities.

2. A high temperature alloy consisting of from 0.6% 1 to 2% titanium, from 0.6% to 2.5% zirconium, from 0.6% to 6% hafnium, from 0.04% to 0.2% oxygen, and the balance essentially niobium with small amounts of incidental impurities.

3. A high temperature alloy consisting of from 0.6% to 2% titanium, from 0.6% to 2.5 zirconium, from 2% to 8% hafnium, from 0.04% to 0.2% oxygen, and the balance essentially niobium with small amounts of incidental impurities.

4. A high temperature alloy consisting of from 2.5% to 3.5% titanium, from 1.5% to 2.5 zirconium, from 1.5% to 2.5% hafnium, from 0.04% to 0.2% oxygen, and the balance essentially niobium with small amounts of impurities.

V 7 References Cited in the file of this patent UNITED STATES PATENTS 2,187,630 Schafer Jan. 16, 1940 2,822,268 HlX Feb. 4, 1958 2,883,282 Wainer Apr. 21, 1959 OTHER REFERENCES Miller: Tantalum and Niobium, Academic Press, Inc., New York, 1959; chapter 11 of interest.

Gonser and Sherwood: Technology of Columbium, John Wiley and Sons, Inc., New York, 1958; pages 

1. A HIGH TEMPERATURE ALLOY CONSISTING OF FROM 0.5% TO 10% TITANIUM, FROM 0.5% TO 10% ZIRCONIUM, FROM 0.5% TO 10% HAFNIUM, FROM 0.02% TO 0.2% OXYGEN, AND THE BALANCE ESSENTIALLY NIOBIUM WITH SMALL AMOUNTS OF INCIDENTAL IMPURITIES. 